This blog provides updated forecasts and comments on current weather or other topics

Friday, November 19, 2010

Uncertainty

Tonight light snow and strong NE winds has spread from the Bellingham area, across the San Juans, to lower Vancouver Island....and the next to be hit is the northern Olympic peninsula. The model forecasts were quite good for this. Here is the current surface observations...you can clearly see the NE winds exiting the Fraser.Saturday and Sunday is really going to be pretty dull around here...not much action, snow-wise or otherwise. And we know that cold is coming and Tuesday morning will be cold enough to produce a hard freeze. The question is snow late Sunday and Monday.

Until this morning, the models showed too little moisture on Monday for any real snow. But now there is some disagreement. The NAM model has a much stronger upper level wave moving down the northerly flow along the West Coast, compared to the GFS model (which is usually the better one). One has a few light snow showers, the other more serious snow. I would generally bet on the GFS model...but we have time to see which solution dominates. Want to see the difference? Here are the two:

Pretty subtle? But a world of difference. There disturbances are forming over the arctic where we don't have that much data. Sometimes we have to live with uncertainty in forecasts and this is one of those times. I expect by Sunday we will know what will happen...enough time to prepare. And I will start using and showing you some of new high-tech tools for dealing with uncertainty...ensembles forecasts with statistical postprocessing!

25 comments:

were it only possible to use those high-tech tools for dealing with uncertainty in non-meteorological areas of one's life...

looking forward to the upcoming posts! the 5 y.o. is still hoping for snow, but now knows we need cold AND "water in the air" (her idea: why don't we take some water from the house and throw it up high to MAKE snow???)

We have around 3 inches on the ground and still snowing like crazy in Bellingham. Take that, snow Grinches! I had to clear about 5 inch drifts from my hood and windshield and navigate crazy slippery roads, some in deep unblemished snow, to get home. Love it! Is that what is meant by light snow? : )

I'm 15 miles NE of Bellingham, and we got only about an inch of snow. Unfortunately the howling north winds have polished that snow into solid ice on the roads, so it's treacherous driving. No sand trucks yet, so I had to bail on my grocery run to town. Melt please!

For a complete weather novice the line "Want to see the difference" made me chuckle because when I look at the two models all I see are "Oh, more pretty blue there than there" and "The pretty squiggly lines move differently." I am going to click on them so they are larger and see if that helps me understand them better. I am fascinated though by the fact that we even have models that can predict weather to begin with. Also, I seem to be the only adult in Seattle rooting for snow in town.

City of Portland seems fairly sure we will have enough snow down here to require clearing it. They've chained up their entire fleet, hung all their plows and their spox said "we're convinced it's coming" (yesterday, on the noon news).

The City uses AccuWeather, which we can't get without a subscription, so AW must have validated the NAM...

Perhaps the best thing to follow along is the "WebAFD": an illustrated version of the National Weather Service Area Forecast Discussion.

In the AFD a forecaster at the NWS (in this case the Seattle office ... SEW) explains his thinking behind the current forecast (once per shift) and possible issues or problems with the forecast. Read the text and look at the images (screen caps from their workstations). You can also see some of the forecaster rules of thumb (the current one is 1000mb to 950mb thickness less than 1300m is cold enough for snow). Even better the AFDs often are humorous.

Of course a good intro to meteorology helps by putting the basics in place. Looking at a pretty plot of the expected vorticity doesn't help if you don't know what vorticity is or why you should care. Or why people would want to know what the layer thickness is.

Hopefully the rush to view the data won't flatten the UW servers. I for one am really grateful that all of this info is published outside of the firewall. And they do take donations too to support their network costs.

I emailed the weather service on Wednesday to let them know they were underestimating the wind speeds for this weekend for the San Juan Islands. NE 10 to 15 mph just didn't cut it. Then they upgraded it on Thursday to 15 - 25 with gusts to 35 as I had suggested. Then they downgraded back to 10-15 mph yesterday morning. And then finally they realized their mistake. Too much now forecasting on wind speeds this past week.